1. Field of the Invention
The present invention relates to an ultraviolet light-emitting device in which a p-type semiconductor is used, and more particularly to an ultraviolet light-emitting device in which a p-type semiconductor having a peak in ultraviolet wavelength region is used.
2. Description of the Related Art
In case of fabricating a semiconductor material, a method for doping semiconductor with impurity wherein both of crystal raw materials of plural types and impurity raw materials of plural types are supplied continuously, whereby a crystal layer formed from the crystal raw materials on a substrate is doped with the impurities in a predetermined apparatus for crystal growth and the like has been heretofore known.
Incidentally, since both the plural types of crystal raw materials and the plural types of impurity raw materials are continuously supplied, impurities of the plural types are incorporated disorderly into atomic layers of crystals formed from the crystal raw materials on the substrate.
As a consequence, there has been such a problem that effect of p-type impurity raw materials cancel that of n-type impurity raw materials (compensation effect) in the case when the above-described plural types of impurities involve p-type impurity raw materials and n-type impurity raw materials, respectively, so that a carrier concentration comes to be a difference in concentration of the p-type impurity raw materials and the n-type impurity raw materials, whereby the carrier concentration of a semiconductor material to be fabricated decreases, and it results in low conductivity.
Furthermore, when both crystal raw materials of plural types and impurity raw materials of plural types are continuously supplied to prepare a p-type semiconductor material in accordance with a conventional method for doping semiconductor with impurity, there has been such a problem that a positive hole of a high density cannot be obtained, so that its conductivity becomes low, since impurity level in forbidden gap of the p-type semiconductor material is deep and activation energy is high.
In view of the problems as mentioned above, the following manners for eliminating such problems have been proposed. Examples thereof include a method and a device for doping a semiconductor with impurities by which a carrier concentration is made to increase even in a case where a crystal layer is doped with p-type impurity raw materials as w 11 as n-type impurity raw materials, whereby a semiconductor material having high conductivity can be prepared; a method and a device for doping a semiconductor with impurities by which a p-type semiconductor material having positive hole of high density and exhibiting high conductivity can be prepared; and a semiconductor material having high conductivity proposed by the present inventors (see Japanese Patent Laid-open No. 2002-75879).
Meanwhile, developments for high conductive p-type AlGaN exhibiting high conductivity are demanded in order to fabricate optical devices as well as electronic devices in ultraviolet wavelength region in late years.
Particularly, a p-type AlGaN material having a high Al composition ratio and exhibiting high conductivity has been indispensable to fabricate ultraviolet light-emitting device such as light-emitting diode (LED) and laser diode (LD) which emits light in ultraviolet wavelength region.
In conventional technology, however, it was difficult to obtain p-type of wide-bandgap nitride semiconductor, and particularly, it was extremely difficult to prepare a high conductive p-type semiconductor which has reached a level wherein the resulting semiconductor may be utilized practically from AlGaN having a high Al composition ratio which is, for example, 20% or higher, i.e. AlxGa1-xN (0.2≦x<1).
In this connection, it was difficult to realize an ultraviolet light-emitting device such as high efficient LED, and high efficient LD in deep ultraviolet wavelength of a wavelength of 330 nm, or shorter.
In other words, difficulty to make ultraviolet light-emitting device such as nitride ultraviolet LED, and nitride ultraviolet LD having short wavelength has been derived from bottleneck of such difficulty to obtain p-type of wide-bandgap AlGaN.
In this connection, since carrier activation ratio of p-type AlGaN exhibits a low value of 1% or less, it is required to dope AlGaN with Mg of a high concentration. Consequently, an important point for making AlGaN to be p-type has been in preparation of high quality AlGaN crystal as a result of doping AlGaN with a high concentration of Mg.